1. Field of the Invention
This invention relates generally to medical devices and especially to intravascular catheters designed to operate with respect to occlusions within a blood vessel. More particularly, this invention relates to catheters for debulking coronary artery lesions and for safely cleaning stents, which are wire frameworks that are compressed, delivered a balloon catheter and positioned across a segment of an artery.
2. Description of the Related Art
Medical science has long sought effective treatments for disease conditions involving stenosis (narrowing or obstruction) of the lumen (interior passage of the artery) of an artery. This condition, known generally as an occlusion, is found in patients suffering from atherosclerosis (accumulation of fibrous, fatty or calcified tissue in the arteries). An occlusion can manifest itself in hypertension (high blood pressure), ischemia (deficiency of circulation), angina (chest pain), myocardial infarction (heart attack), stroke, or death. An occlusion may be partial or total, may be soft and pliable or hard and calcified, and may be found at a great variety of sites in the arterial system including the aorta, the coronary and carotid arteries, and peripheral arteries.
Of particular interest to cardiac medicine are the often disabling or fatal occlusions occurring in the coronary arteries (arteries supplying the heart). Traditionally, coronary artery occlusions have been treated by performing coronary bypass surgery, in which a segment of the patient's saphenous vein is taken from the patient's leg and is grafted onto the affected artery at points proximal (upstream) and distal (downstream) to the occluded segment. The bypass often provides dramatic relief. However, it entails dangerous open chest surgery and a long, painful, costly convalescence in the hospital. Moreover, with the passage of time, the bypass patient's saphenous vein graft can also become occluded. If the patient has another saphenous vein, a second bypass procedure may be performed, once again entailing open chest surgery and prolonged hospitalization. Thereafter, if the underlying atherosclerotic disease process is not controlled, the prognosis is dismal.
Newer, minimally invasive procedures are now preferred in the treatment of arterial occlusions. These procedures use a catheter, a long, thin, highly flexible device which is introduced into a major artery through a small arterial puncture made in the groin, upper arm, or neck and is advanced and steered into the site of the stenosis. At the distal end of the catheter, a great variety of miniature devices has been developed for operating upon the stenosed artery.
The more popular minimally invasive procedures include percutaneous transluminal coronary angioplasty (PTCA), directional coronary atherectomy (DCA), and stenting. PTCA employs a balloon to mechanically dilate the stenosis. In PTCA, a steerable guidewire is introduced and advanced under fluoroscopic observation into the stenosed artery and past the stenosis. Next, a balloon-tipped catheter is advanced over the guidewire until it is positioned across the stenosed segment. The balloon is then inflated, separating or fracturing the atheroma (stenosed tissue). The hoped-for outcome is that, over time, the lumen will stay open.
In directional coronary atherectomy a catheter, containing a cutter housed in its distal end, is advanced over the guidewire into the stenosed segment. The housing is urged against the atheroma by the inflation of a balloon, so that part of the atheroma intrudes through a window in the side of the housing. Under fluoroscopic observation, the cutter is used to shave away the atheroma. The shavings are collected in the nosecone of the housing and withdrawn along with the catheter or flushed out of a flushing lumen running the length of the device.
Some examples of existing devices include U.S. Pat. No. 5,074,841 to Ademovic et al.; U.S. Pat. No. 4,669,469 to Gifford III et al.; and the curretting tool described in U.S. Pat. No. 4,867,157 to McGurk-Burleson et al. The devices described in these references, however, are solely optimized toward uses involving helical cutting blades, the cutting and grinding of flap-like pieces of atheromas, or a penetration into the patient's body of only a few centimeters. None of the devices disclosed in these patents, however, is effective in stented portions of the patient's vessels.
Stenting is a procedure in which a wire framework, known as a stent, is compressed and delivered a balloon catheter. The stent is positioned across the stenosed segment of the artery. The balloon is inflated, dilating the stent and forcing the stent against the artery wall. The hoped-for outcome is that the stent will hold the arterial lumen open for a prolonged period. Frequently, a stent is placed in an artery immediately following PTCA or DCA.
However, over time, the stent itself may become stenosed, as fatty or calcified tissue accumulates in and around the wire mesh or struts of the stent. This can happen for several reasons. One reason for this accumulation may be intimal hyperplasia which leads to tissue ingrowth within the wire mesh of the stent, resulting in in-stent re-stenosis. Another reason for the re-stenosis of the stented area may be a compliance mismatch between stented and non-stented areas. The type of and materials used for the stent may also lead to an auto-immunological response wherein the patient's body recognizes the stent as a foreign body and attempts to reject it. Antibodies may then cling to the wire mesh of the stent to such a degree as to eventually occlude the vessel and constrict blood flow. Still another reason may be that the patient has maintained the same lifestyle and eating habits as before the stenting procedure. Therefore, the same factors that lead to the original occlusion persist, and the arteries suffer from the same fatty deposits and plaque that created the first stenosis.
In these cases, and for whatever the underlying reason, fatty tissue and plaque grow in the interstitial spaces in the wire mesh of the stent, and eventually grow outside the stent to occlude the very vessel the stent was designed to maintain open. It is estimated that this in-stent re-stenosis occurs, for single stents in a discrete focal region, in approximately 12 to 20 percent of stenting cases. The re-stenosis rates for diffusely diseased vessels, which often require more than one stent, have been reported as high as 80 percent.
Currently, there is no effective method of treating in-stent re-stenosis. For example, one proposed solution involves the application of another stent to the occluded area of the first stent. In effect, this amounts to re-stenting the first stent. This procedure, however, is not an optimal solution, as the blood vessel may not support two stents, and re-stenting does not address the underlying problem of accumulated plaque or other fibrous material lodged within the stent wire mesh.
Another proposed solution to the problem of in-stent re-stenosis involves the use of balloon angioplasty. However, such a method may not be indicated in some applications, due to high re-stenosis rates. Indeed, balloon angioplasty has been reported to be ineffective due to the soft nature of some lesions, which mainly consist of smooth muscle cells migrating to form intimal hyperplasia. Balloon dilation also causes the tissue to extrude out into non-dilated areas, narrowing the lumen diameter and causing turbulent flow in such sites. Such turbulent flow is thought to be a contributing factor in the development of vascular occlusions. In addition, further dilation may cause further expansion of the stent, thereby causing more vessel injury due to stent strut/vessel interaction, which interaction may have contributed to the re-stenosis in the first place. Indeed, PTCA for in-stent re-stenosis has shown re-stenosis rates as high as 55 percent, even when the Mean Lumen Diameter (MLD) after dilation is within 3 to 5 percent of the original values.
The use of lasers, such as ELCA, has also been proposed, but lasers are expensive and limited in the amount of debulking that can be achieved.
What is desired, therefore, is a device for cleaning stents that have become occluded, to thereby remove the unwanted material and restore the full intended therapeutic effects of the arterial stent.
However, using a conventional atherectomy catheter to clean a stenosed arterial stent may not be the most effective therapy, as portions of the wire mesh of the stent may become invaginated along with the stenosed tissue. This undesirable invagination of the stent material can cause entanglement or cutting of the mesh in the atherectomy catheter cutter, or even the collapse of the stent. Even if the stent does not collapse, the wire mesh can become wound about the cutter element. To retract the catheter, thereafter, it is necessary to remove it, together with the entangled stent, from the patient's body, with potentially undesirable outcomes. Other undesirable outcomes of cutting a portion of the stent may occur if the inadvertently cut portion of the stent is not collected by the catheter and, instead, travels back to the cardiac muscle.
What is desired, therefore, is a device for the controlled invagination of the occluding material at the stent location without, however, running the risk of invaginating a portion of the wire mesh of the stent along with the stenosed tissue.
It is not only the stent material that can become undesirably invaginated in the cutter opening of the catheter. In some cases, the vessel walls can also be invaginated and cut by the cutter. In those cases, the vessel can be damaged, necessitating complex and more invasive repair work on the part of the physician, with accompanied increased risk for the patient. There has been a long felt need, by physicians and patients alike, for an atherectomy catheter which minimizes the probability of undue vessel damage during the DCA procedure proper.
What is desired, therefore, is an atherectomy catheter which will allow the physician to perform optimal atherectomy, that is, atherectomy which prevents the vessel walls from being damaged or cut by the working element during the DCA procedure. Specifically, what is desired is an atherectomy catheter which allows the undesirable stenotic material to become invaginated into the cutter or working element opening, but prevents the walls of the vessel from being so invaginated, cut or damaged by the cutter.
Moreover, in conventional atherectomy catheters equipped with a reciprocating or rotary cutter or abrasive working head or element, it is conceivable that the cutter or working element may break away from the cutter torque cable. Also, ejection of the cutting element from the catheter housing could occur because of, for example, a microfracture in the cutter, especially when such microfractures undergo the high magnitude stresses imposed by the extremely high rotational speeds of modern atherectomy catheter cutters, which speeds are on the order of 10,000 rpm.
What is desired, therefore, is a device for preventing damage to the vessel walls when the working element, such as a cutter or an abrasive element of an atherectomy catheter, accidentally becomes dislodged from the catheter housing.
In addition, in conventional catheters, there can occur a twisting or torquing of the housing during use, due to the high rotational speeds of the working element and the reciprocal motion imposed thereupon. The twisting of the housing then generates a corresponding twisting of the housing opening within the lumen. This occurs despite the presence of the balloon which is designed to keep the catheter housing stationary during use. Such twisting is undesirable, as the orientation of the housing opening determines the location of the cuts or abrasions carried out by the working element.
What is desired, therefore, is an atherectomy catheter which has greater structural resistance to forces which tend to twist the catheter housing during use.
Debulking a particular site of a coronary artery is a useful procedure often performed before the placement of a stent. This procedure provides a larger cross sectional area within which the stent can be implanted. Debulking also removes most of the lesion which would otherwise extrude to outside the stent/balloon boundaries. Debulking is also a very useful procedure in the case of eccentric lesions. In eccentric lesions, stents are more apt to expand into the healthier, more elastic side of the lumen, thereby causing injury to the healthier side of the artery. However, conventional debulking atherectomy catheters may damage the arterial walls, as a portion of the vessel walls become invaginated into the housing opening.
What is desired, therefore, is an atherectomy catheter which can be used for debulking arterial lesions but which, nevertheless, provides optimal protection against an undesired invagination of vessel wall into the housing opening.